1. Field
The present invention relates generally to video processing, and more specifically, to methods and systems for providing bi-directionally predicted video coding.
2. Background
The advent of high-speed, low-cost microprocessors has prompted a rapid growth in the development of devices that enable efficient storage and transmission of digital audio and video data. The huge demand for such devices, along with the desire to improve the quality of stored and/or transmitted audio and video data in systems with limited storage and transmission capacities, has resulted in rapid advancement of video data compression techniques. Furthermore, the typical large size and inherent redundancy of video files have also incentivized the development of such techniques.
Several ISO/ITU standards on video compression exist including, for example, MPEG1, MPEG2, MPEG4, H.263, and H.264. These standards exploit the spatio-temporal correlation in the video data, along with entropy-coding techniques, to achieve high compression ratios. These standards define three basic types of coded images or pictures including Intra predicted (I), Inter predicted (P) and Bi-directionally predicted (B) pictures. As B pictures employ motion-estimation-based prediction from past as well as future reference pictures, the coding efficiency of B pictures is in general superior to that of the other two types of pictures mentioned above.
The existing ISO/ITU video coding standards provide several methods (modes) for encoding bi-directionally predicted B pictures. The basic modes include a forward prediction mode, a backward prediction mode, a bi-directional prediction mode and a direct prediction mode. While the first three modes mentioned above require coding of all the motion vector data of the predicted picture with respect to a reference picture or pictures, the direct prediction mode requires coding of either a very small amount of motion vector data or, in some cases, no motion vector data at all. Several variants of the direct prediction mode appear in different video coding standards. In these various direct prediction modes, the forward and backward motion vectors are derived from the motion vectors that have already been computed between the reference pictures. These various direct prediction modes are thus particularly attractive because their implementation is computationally less involved and they are highly compressible. FIG. 1 illustrates a direct mode of the MPEG4 standard.
The latest video coding standard, the H.264 (ISO/IEC 14496-10), achieves much higher coding efficiency compared to previous standards. This is accomplished mainly due to better exploitation of the spatio-temporal correlation that may exist between picture segments. The main tools introduced in H.264 for this purpose are variable block sizes for motion compensation, multiple reference frames, generalized B images, multiple hypothesis prediction, intra prediction, and two direct modes for bi-directionally predicted images. The importance of direct modes is even higher in H.264 since it allows motion-estimation-based prediction at a much smaller block level. FIG. 2 illustrates a direct mode of the H.264 standard.
The usefulness of conventional direct modes, however, is limited to the case where the motion of picture segments in the bi-directionally predicted B picture is linear and of low intensity. These modes employ scaled motion vectors between reference pictures for predictive coding. Besides, in the conventional direct modes, the reverse motion vectors used for backward prediction are simply the scaled negative values of the forward motion vectors. This is a crude approximation and yields poor predictions that result in lower coding efficiency. The coding efficiency of direct modes is further reduced when motion vectors are not available in the reference picture(s), as in the case of Intra segments in the reference pictures.
Hence, it would be desirable to provide methods and systems that are capable of providing bi-directionally predicted video coding in a more efficient manner.